I've been using Event.addBehavior and recently moved to 0.5.  I found
this note by Dan

"Event.addBehavior.reassignAfterAjax is now false by default:
Normally, if you are relying on this behavior it's much more efficient
to move to a solution using event delegation. However, if you do want
your behaviors reassigned to new content after Ajax calls then go
ahead and set it back to true again."

I did set Event.addBehavior.reassignAfterAjax to true and everything
worked again, but I'm curious as to how to use the more efficient
method.  I have a lot of code like this:

'a.img img:mouseover': function() {
//

how would I move that to event delegation?

thanks